What is the most efficient way to make Concrete from Concrete Powder?

Another way to achieve this without the need for mining the concrete, would be to make use of the fact, that TNT now drops all the blown up blocks.

So you could insert the powder into a piston feed tape, which is next to water => powder gets converted to concrete.

The stream of concrete get blown up by TNT after that. With a simple water stream collection system in the end you can build something that works like a charm.

If you search for TNT blast chamber you will most likely find something that works wonders.

Disclaimer: Many of the designs out there make use of a bug [Java version 1.15] in the game to duplicate the TNT block that is used to blow everything up. I don't think that this will work for bedrock, but if you have enough gun powder you should be able to achieve the same result with normal TNT